Method of making porous nitrogenized titanium coatings for medical devices

ABSTRACT

The disclosure describes a method of making porous nitrogenized titanium coated substrates. The process includes depositing titanium on a substrate using a glancing angle deposition (GLAD) process and then nitrogenizing the deposited titanium using a thermal or plasma-assisted thermal gaseous nitrogenizing process.

FIELD

The disclosure relates processes for making porous nitrogenized titanium coatings for use in medical devices.

BACKGROUND

Titanium nitride (TiN) films deposited by physical vapor deposition (PVD) can exhibit desirable properties for implantable medical device electrodes. The most desirable TiN coatings for electrode applications are typically relatively thick (2-20 um) and have a porous, columnar, microstructure which exhibits high specific surface area and high specific capacitance.

Formation of such porous, TiN columnar structures depends upon maintaining low add-atom mobility during the deposition process. Specifically, the fabrication of porous, controlled microstructures by sputter-deposition processes requires deposition process operation at relatively high working gas pressures, leading to low deposition rates. Such sputter-deposition processes result in long deposition times to achieve the desired 2-20 μm film thickness and therefore high cost.

SUMMARY

The disclosure provides methods of treating surfaces or substrates using glancing angle deposition and nitrogen to produce substrates having porous surfaces having increased surface area and capacitance as compared to an untreated surface.

In one example, a method described in this application comprises providing a substrate having a surface, depositing a porous titanium film on the surface of the substrate using glancing angle deposition, and converting at least a portion of the porous titanium film to a porous film comprising nitrogenized surface layers using a thermal or plasma-assisted thermal gaseous nitrogenizing process.

The methods described in this application produce nitrogenized porous titanium films that are substantially free of titanium nitride (TiN).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a glancing angle deposition process (GLAD) and its geometry.

FIG. 2 is schematic depiction of a plasma-assisted gaseous thermal treatment system used for nitrogenizing certain substrates in the Examples below.

FIG. 3 is a schematic depiction of a turbo-molecular-pumped quartz-tube vacuum furnace system used for nitrogenizing certain substrates in the Examples below.

FIG. 4 a is a scanning electron microscope image of Example 1 after GLAD.

FIG. 4 b is a scanning electron microscope image of a flaw shown in FIG. 4 a.

FIG. 4 c is a scanning electron microscope image showing the as deposited GLAD film morphology of Example 1 in cross-section following fracture of the film.

FIG. 4 d is a scanning electron microscope image of Example 1 in cross section after GLAD and nitrogenizing.

FIG. 5 shows a plot of the results of electrochemical impedance measurements as a function of frequency for bare titanium, titanium treated using GLAD, and titanium treated using GLAD and then nitrogenized.

FIG. 6 shows a plot of the results of first cyclic voltammetry measurements comparing titanium treated using GLAD and titanium treated using GLAD and then nitrogenized.

FIG. 7 shows a plot of the results of the first and third voltammetry cycle measurements comparing titanium treated using GLAD and titanium treated using GLAD and then nitrogenized.

FIG. 8 shows X-ray diffraction data obtained from a nitrogenized titanium coupon treated according to the nitrogenizing processes of the disclosure.

DETAILED DESCRIPTION

Applicants have discovered that substrates having a porous titanium layer deposited onto the substrate using a glancing angle deposition process (GLAD) followed by thermal or plasma-assisted nitrogenizing of the porous titanium layer provides articles useful in medical devices, for example electrodes. The resulting article made by the above processing steps can be produced at a higher rate and can be produced at lower cost than can be made using other known processes. The nitrogenized porous titanium layer is substantially free of TiN, that is, free of detectable levels of TiN as determined by X-ray diffraction (XRD).

In general, a GLAD deposition process comprises, consists of or consists essentially of a process in which vapor deposited porous films are deposited on a substrate by directing a vapor flux at an oblique angle to the substrate. The substrate can be rotated at an axis normal to the substrate during the deposition process. The rotation can be at a set rate of slow continuous RPMs or can be paused after a partial rotation for a period of time then rotated, then paused, etc. or until a desired film thickness is obtained.

FIG. 1 schematically depicts a GLAD process system and its geometry. GLAD process system 10 comprises an evaporant source 12 and a substrate holder 14 mounted on a distal end of a rotary feedthrough 16, all of which are housed within a vessel 18. The evaporant source 12 comprises the metal or substance that forms a vapor when impinged by an electron beam (not shown). The evaporant 12 (for example, titanium) and substrate holder 14 are oriented within the vessel 18 such that a normal line 20 from the evaporant is perpendicular to a normal line 22 from the substrate holder 14, and of course a substrate once positioned within the holder.

The incidence angle “α” is defined as the angle between the substrate holder and substrate surface normal 22 and the substantially line-of-sight path followed by the impinging evaporant flux from evaporant source 12 to the substrate surface. GLAD process system 10 further comprises deposition rate monitor 24 and moveable shutter 26. Deposition rate monitor 24 samples a fraction of the evaporant flux emitted from evaporant source 12 and provides feedback to evaporant source 12 such that a desired deposition rate can be established and maintained. Moveable shutter 26 can be positioned to control the fraction of evaporant flux emitted from evaporant source 12 reaching the substrate.

In the GLAD process described in this application, useful incidence angles include from about 65 degrees to about 85 degrees, including any range or value in between the range of about 65 degrees to about 85 degrees.

Useful substrates to be coated include those made from titanium, platinum, niobium, tantalum and alloys of any of them. Other materials capable of withstanding the temperatures associated with the subsequent nitrogenizing may be suitable for some applications.

In general, a thermal or plasma-assisted gaseous nitrogenizing process comprises or consists of or consists essentially of exposing a substrate within a vacuum furnace, with or without a plasma discharge, to nitrogen an elevated temperature and pressure for a time period until the desired nitridization occurs. Typically, the nitrogenizing process occurs in an oxygen and air free environment which is provided by first evacuating the vessel and then purging the vessel with nitrogen, argon or both. In this disclosure, the porous metal films are nitrogenized to form porous metal films comprising, having, consisting of, or consisting essentially of nitrogenized or nitrogen-enriched surfaces or surface layers. “Consisting essentially of” nitrogenized surfaces means surfaces substantially free of TiN.

Typically, the nitrogenizing process takes place for a time not exceeding about 2 hours at a temperature of from 650° C. to 750° C. at a pressure of less than 1×10⁻³ Pa. The temperature range of from 650° C. to 750° C. is intended to support any range or value between 650° C. to 750° C.

Methods described in this disclosure provide nitrogenized porous films that have increased capacitance and increased surface area when compared to untreated substrates as determined by the shift in impedance corner frequency. The shift in impedance corner frequency directly corresponds to a change in double-layer capacitance. Double-layer capacitance is directly proportional to surface area.

In one embodiment, the method including GLAD and nitrogenizing results in an increase in surface area (as measured by electrochemical impedance measurements, that is, the shift in impedance corner frequency) by a factor of 10 or greater as compared to the untreated geometric surface area of the substrate. In another embodiment, the method results in an increase in surface area (as measured by the shift in impedance corner frequency) by a factor of 100 or greater as compared to the untreated geometric surface area of the substrate. In another embodiment, the method results in an increase in surface area (as measured by the shift in impedance corner frequency) by a factor ranging from 10 to 100, inclusive and including any range or value within the range of 10 to 100.

EXAMPLES 1-7 GLAD Process

The GLAD process system geometry used is shown schematically in FIG. 1. Additional detailed processing parameters used in the examples are shown below in Table 1.

A grade 1 titanium sheet (25×50×0.18 mm) was prepared by cleaning in a hot deionized water/detergent mixture followed by multiple deionized hot water rinses. The titanium sheet was used as a substrate on which titanium in the form of a porous layer was deposited in a TEMESCAL electron beam evaporator (available from Temescal, Livermore, Calif.) by glancing angle deposition (GLAD). After mounting the substrate on the rotary substrate holder, the system was closed and evacuated to a pressure of <2.5 10⁻³ Pa. Power was then slowly increased to the electron beam evaporant source until the desired evaporation rate was established with the moveable shutter closed. Once the desired evaporation rate was established and stabilized, the moveable shutter was opened and deposition on the substrate commenced.

Each sample was rotated approximately 90° after a desired incremental film thickness was deposited (100 nm or 200 nm); the approximately 90° rotations were repeated after each incremental deposition until the desired total film thickness was obtained. The specific values of for deposition rate, incremental film thickness and total film thickness along with other process parameters are shown in Table 1. Upon achieving the desired total film thickness the evaporation source was shut down, the system was allowed to cool and was then vented to atmosphere.

The samples were then nitrogenized using either Nitrogenizing Process 1 or Nitrogenizing Process 2, described in more detail below. The nitrogenizing process temperature was 700° C. for all examples described herein. Process time is defined herein as the time from introduction of the nitrogen gas to the beginning of the furnace cooling cycle.

Nitrogenizing Process 1: Nitrogenizing process 1 was performed in a plasma-assisted thermal treatment system. As shown schematically in FIG. 2, the plasma assisted thermal treatment system 100 consisted of a quartz tube vacuum furnace 102, scroll vacuum pumping system 103, and inductively-coupled RF coil 104. Mass flow controllers 106, 108 were used to control the inlet flow of Ar and N₂ process gases 105, 107, and a downstream throttle valve 110 was employed on the quartz tube furnace to control gas pressure during the thermal nitrogenizing process. The process gas purity was 99.99% or better. Nitrogenizing process 1 included the following process steps:

-   1. Each sample was loaded into a titanium furnace boat and evacuated     to a pressure of <0.4 Pa. -   2. The argon flow rate was set to 15 sccm and the furnace was heated     to a temperature of 700° C. The furnace heating time was     approximately 30 minutes. -   3. The throttle valve was set to maintain argon pressure of     approximately 3.3 Pa and the plasma was ignited at a level of 100     watts. -   4. The argon plasma was ignited for 10 minutes as a cleaning step. -   5. The nitrogen flow rate was set at 15 sccm to begin the     nitrogenizing process. The process pressure of nitrogen was     maintained at about 3.3 Pa and the RF plasma remained ignited for     the duration of the 2 hour nitrogenizing process. -   6. After nitrogenizing of the samples was completed, the furnace and     the RF power were shut down while the argon and nitrogen gas flows     and 3.3 Pa pressure were maintained until the furnace cooled to a     temperature of <50° C. After the furnace was cooled, the furnace was     vented to the atmosphere and the samples were removed.     Nitrogenizing Process 2: Nitrogenizing process 2 was performed in a     turbo-molecular-pumped quartz-tube vacuum furnace 200 which is shown     schematically in FIG. 3. Turbo-molecular-pumped quartz-tube vacuum     furnace 200 comprised quartz tube furnace 202, nitrogen gas inlet     203 and a micrometer valve 204 to control the inflow of nitrogen. A     turbomolecular vacuum pump 206 in series with scroll vacuum pump 208     was used to evacuate the system. The nitrogen gas purity was better     than 99.99%. Nitrogenizing process 2 included the following process     steps: -   1. 1. Each sample was loaded into a titanium furnace boat and     evacuated to a pressure of <1×10⁻³ Pa. -   2. The furnace was heated to a temperature of 700° C. at a rate of     20° C./minute. -   3. The turbo-molecular pump was shut down while the scroll vacuum     pump remained on and the nitrogen gas flow was maintained at a     pressure of about 4000 Pa. -   4. The nitrogen gas flow was maintained during the 2 hour     nitrogenizing process and until the furnace cooled to a temperature     of <50° C. After the furnace cooled, the furnace was vented to the     atmosphere and the nitrogenized samples were removed.

TABLE 1 Deposition Flux 90 Degree Deposi- Indicated Incidence Rotation tion Nitro- Exam- Thickness angle, α Period Rate genizing ple (nm) (degrees) (nm) (nm/s) Process 1 2000 71.5 100 1 1 2 2000 80 200 2 1 3 2000 77 200 2 1 4 4000 74.5 100 2 1 5 2000 71.5 100 1 2 6 2000 80 200 2 2 7 2000 77 200 2 2

Electrochemical impedance spectroscopy was used to compare the impedance magnitude as a function of frequency for uncoated titanium sheet to both as deposited and nitrogenized GLAD films. The measurements were performed using a SOLARTRON SI1287 Electrochemical Interface and SI1260 Impedance Gain-Phase Analyzer (Available from Solartron Analytical Division of Ametek, Farnborough, Hampshire, United Kingdom). The measurements were performed in phosphate-buffered saline solution. The AC excitation voltage was 100 mV, and the frequency range examined was 10,000 to 0.1 Hz. A platinum foil with an exposed surface area of approximately 14 cm² was used as a counter electrode. Exposed surface area on the measured specimens was approximately 0.3 cm².

Cyclic voltammetry was also performed using the identical experimental setup described for the impedance spectroscopy with the addition of a reference electrode. The reference electrode used was Ag/AgCl in 3M NaCl. Potentials were cycled from −1.5 to +1 volts relative to the reference electrode at a scan rate of 25 mV/s. Three cycles were typically performed. Changes in behavior observed for more than three cycles were minimal.

Results

After the samples were deposited with titanium using the above described

GLAD process, all of the samples appeared flat-black in color. The samples exposed to Nitrogenizing process 1 described above were turned light brown in color.

FIG. 4 includes SEM images of the undisturbed film surface as well as film fracture surfaces obtained by folding the substrate upon itself (˜180 degree bend angle).

FIG. 4 a is an SEM image (500×) of Example 1 after GLAD but before Nitrogenizing Process 1 as viewed substantially normal to the sample surface.

FIG. 4 b is an SEM image (10,000×) of a flaw shown in FIG. 4 a as viewed substantially normal to the sample surface.

FIG. 4 c is an SEM image (30,000×) showing the as deposited GLAD film morphology of Example 1 in cross-section following fracture of the film as described above. The morphology comprises small, partially isolated columns which exhibit a “kinked” structure due to the stepwise rotation during deposition.

FIG. 4 d is an SEM image (30,000×) of Example 1 in cross section after the GLAD process and Nitrogenizing Process 1 described above.

The highly porous nature of the resulting structure is evident from FIGS. 4 c and 4 d. Impedance spectra of an example 1 specimen before and after Nitrogenizing Process 1 are compared to the titanium substrate material with no coating in FIG. 5. The shift in corner frequency by more than two orders of magnitude in frequency demonstrates that surface area and hence double-layer capacitance has also increased by more than two orders of magnitude for the GLAD examples. The data show a greater than 100 fold increase in double-layer capacitance which is directly proportional to surface area. The intersection of the horizontal lines and the tangents lines is the corner frequency.

FIG. 6 compares the first voltammetry cycle for an exemplar GLAD film in the as deposited state and after Nitrogenizing Process 1. The nitrogenizing process significantly reduces the currents at both the anodic and cathodic potential extremes. FIG. 7 compares the first and third voltammetry cycle for an exemplar GLAD film in the as deposited state and after Nitrogenizing Process 1 with the scale expanded relative to FIG. 6; it shows that Nitrogenizing Process 1 also suppresses secondary electrochemical reactions that occur on the as deposited film.

EXAMPLES 8-10

Flat titanium samples were treated using GLAD as described above. The GLAD treated samples were nitrogenized using Nitrogenizing Process 2 for two hours at 650° C. (Example 8), 750° C. (Example 9), and 850° C. (Example 10). Each sample nitrogenized at one of the three temperatures was analyzed using XRD. No additional phase formation was shown at 650° C. The sample nitrogenized at 750° C. showed that the presence of Ti₂N was predominating, with a small peak attributable to TiN. The sample nitrogenized at 850° C. showed more of both Ti₂N and TiN as compared with the sample nitrogenized at 750° C.

EXAMPLE 11

X-ray diffraction (XRD) was performed on a grade 1 titanium sheet sample nitrogenized as described in Nitrogenizing Process 2. The measurements were performed using a PANalytical Xpert mpd system (available from PANalytical B.V., Lelyweg 1, 7602 EA ALMELO, The Netherlands). The XRD system employed a Cu target X-ray source operated at 40 KV, 40 mA. The incident radiation was filtered by a nickel filter and the system employed an Xcellerator multi-channel detector. The scan range was 30-80 degrees 2Θ, with a 2Θ step size of 0.02 degrees. Measurement time per step (multichannel data collection) was 163 seconds.

As shown in FIG. 8, Ti2N phase formation is observed but substantially no TiN was detected. Three small peaks were attributable to Ti₂N. FIG. 8 is a reproduction of the XRD plot of peak position (2° Theta) vs number of counts. Peak positions marked “A” indicate Ti₂N; Peak positions marked “B” indicate TiN; and the largest peaks indicate Ti.

Thus, embodiments of the methods of forming porous nitrogenized titanium films are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow. 

What is claimed is:
 1. A method comprising: providing a substrate having a surface; depositing a porous titanium film on the surface of the substrate using glancing angle deposition; and converting at least a portion of the porous titanium film to a porous film comprising nitrogenized surface layers using a thermal or plasma-assisted thermal gaseous nitrogenizing process.
 2. The method according to claim 1, wherein the substrate is an electrode.
 3. The method according to claim 1, wherein the substrate comprises platinum or titanium or alloys of either.
 4. The method according to claim 1, wherein the porous titanium film is deposited on the surface of the substrate at a thickness of at least 1 micrometer.
 5. The method according to claim 1 wherein the substrate is a metal.
 6. The method according to claim 1 wherein the surface area, as determined by electrochemical impedance measurements is increased by a factor of 100 or more relative to the geometric surface area of the substrate.
 7. The method according to claim 1 wherein the surface area, as determined by electrochemical impedance measurements is increased by a factor of 10 or more relative to the geometric surface area of the substrate. 